274 THE GEOLOGICAL SUCCESSION [CHAP. XL 



during long and equal periods of time, may, perhaps, be nearly the 

 same; but as the accumulation of enduring formations, rich in 

 fossils, depends on great masses of sediment being deposited on 

 subsiding areas, our formations have been almost necessarily accu- 

 mulated at wide and irregularly intermittent intervals of time ; con- 

 sequently the amount of organic change exhibited by the fossils 

 embedded in consecutive formations is not equal. Each formation, 

 on this view, does not mark a new and complete act of creation, 

 but only an occasional scene, taken almost at hazard, in an ever 

 slowly changing drama. 



We can clearly understand why a species when once lost should 

 never reappear, even if the very same conditions of life, organic 

 and inorganic, should recur. For though the offspring of one 

 species might be adapted (and no doubt this has occurred in in- 

 numerable instances) to fill the place of another species in the 

 economy of nature, and thus supplant it ; yet the two forms the 

 old and the new would not be identically the same ; for both 

 would almost certainly inherit different characters from their 

 distinct progenitors ; and organisms already differing would vary 

 in a different manner. For instance, it is possible, if all our fan- 

 tail pigeons were destroyed, that fanciers might make a new breed 

 hardly distinguishable from the present breed ; but if the parent 

 rock-pigeon were likewise destroyed, and under nature we have 

 every reason to believe that parent-forms are generally supplanted 

 and exterminated by their improved offspring, it is incredible that 

 a fantail, identical with the existing breed, could be raised from 

 any other species of pigeon, or even from any other well-established 

 race of the domestic pigeon, for the successive variations would 

 almost certainly be in some degree different, and the newly-formed 

 variety would probably inherit from its progenitor some character- 

 istic differences. 



Groups of species, that is, genera and families, follow the same 

 general rules in their appearance and disappearance as do single 

 species, changing more or less quickly, and in a greater or lesser 

 degree. A group, when it has once disappeared, never reappears ; 

 that is, its existence, as long as it lasts, is continuous. I am aware 

 that there are some apparent exceptions to this rule, but the 

 exceptions are surprisingly few, so few that E. Forbes, Pictet, and 

 Woodward (though all strongly opposed to such views as I main- 

 tain) admit its truth ; and the rule strictly accords with the theory. 

 For all the species of the same group, however long it may have 

 lasted, are the modified descendants one from the other, and all 

 from a common progenitor. In the genus Lingula, Tor instance, 

 the species which have successively appeared at all ages must have 

 been connected by an unbroken series of generations, from the 

 lowest Silurian stratum to the present day. 



